1. Field of Invention
This invention relates to the propagation of electromagnetic waves and more particularly to monopulse radar.
2. Description of the Prior Art
A multimode feed of a monopulse radar typically includes a waveguide with a cavity of a large cross-sectional size and a waveguide with a cavity of a small cross-sectional size. The ends of the larger waveguide are respectively connected to an end of the smaller waveguide and the input of a horn radiator.
The difference between the cross-sectional sizes causes the connection between the waveguides to form what is known as a discontinuity. Because of the discontinuity, when an electromagnetic wave propagates in a TE.sub.10 mode through the smaller waveguide towards the larger waveguide, electromagnetic waves propagate through the larger waveguide in the TE.sub.10 mode and higher order modes. The size of the large waveguide is selected to prevent a propagation of a wave in a TE.sub.30 mode or a mode of higher order than the TE.sub.30 mode.
One of the higher order modes of propagation within the larger waveguide is an LSE.sub.12 mode. The waves that propagate through the larger waveguide in the LSE.sub.12 and TE.sub.10 modes (referred to as sum mode waves hereinafter) comprise an electromagnetic wave that propagates in the sum mode of the monopulse radar.
Usually, the larger waveguide has a length that causes the sum mode waves to have a phase change of 360 degrees relative to each other when they propagate from the discontinuity to the aperture of the horn. Because of the 360.degree. phase change, there is a desired relative phase at the aperture between the sum mode waves.
One undesired aspect of the multimode feed is that a deviation of the frequency of excitation of the multimode feed from a center frequency causes a directly related deviation of the phase change from 360 degrees. When the deviation of the phase change is large, the radar is inoperative. Therefore, the deviation of the phase change is a limitation on the bandwidth of the multimode feed.
In addition to the sum mode waves, there are typically electromagnetic waves that propagate through the layer waveguide in an LSE.sub.11 mode and TE.sub.20 mode. As known to those skilled in the art, the LSE.sub.11 and TE.sub.20 modes are the difference modes of the radar.
Another undesired aspect of the multimode feed is that it usually causes the polarization of the waves that propagate in the LSE.sub.11 and TE.sub.20 modes to have a component that is orthogonal to the polarization of the sum mode waves. When the radiator causes the orthogonal polarization, the multimode feed is said to depolarize the wave that propagates in the LSE.sub.11 and TE.sub.20 modes. A processing of a signal derived from a depolarized electromagnetic wave may provide erroneous data relating to the azimuth and elevtion of a target.